Insert into getCopyDataMessage() the same logic that already existed in the

main code path for enlarging libpq's input buffer in one swoop when needing to
read a long data message.  Without this, the code will double the buffer size,
read more data, notice it still hasn't got the whole message, and repeat till
it finally has a large enough buffer.  Which wastes a lot of data-moving
effort and also memory (since malloc probably can't do anything very useful
with the freed-up smaller buffers).  Not sure why this wasn't there already;
certainly the COPY data path is a place where we're quite likely to see long
data messages.  I'm not backpatching though, since this is just a marginal
performance issue rather than a real bug.

src/interfaces/libpq/fe-protocol3.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/interfaces/libpq/fe-protocol3.c,v 1.33 2008/01/15 22:18:20 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/interfaces/libpq/fe-protocol3.c,v 1.34 2008/01/17 21:21:50 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1305,7 +1305,24 @@ getCopyDataMessage(PGconn *conn)
 		}
 		avail = conn->inEnd - conn->inCursor;
 		if (avail < msgLength - 4)
+		{
+			/*
+			 * Before returning, enlarge the input buffer if needed to hold
+			 * the whole message.  See notes in parseInput.
+			 */
+			if (pqCheckInBufferSpace(conn->inCursor + msgLength - 4, conn))
+			{
+				/*
+				 * XXX add some better recovery code... plan is to skip over
+				 * the message using its length, then report an error. For the
+				 * moment, just treat this like loss of sync (which indeed it
+				 * might be!)
+				 */
+				handleSyncLoss(conn, id, msgLength);
+				return -2;
+			}
 			return 0;
+		}
 
 		/*
 		 * If it's a legitimate async message type, process it.  (NOTIFY